The present invention pertains to a method for the production of a polyester multifilament yarn.
High-strength filaments of polyethylene terephthalate (PET) are used, as known, in various facets of industry, e.g., for the manufacture of tire cord.
The invention in particular pertains to a polyester (PET) that contains at least 90 mol % of ethylene terephthalate.
For the use of PET in the above-identified areas, yarns with particularly stable molecular structures are needed. U.S. Pat. No. 4,195,052 describes a method for the production of high-strength PET yarns with low shrinkage and little heat generation under cyclical, mechanical loads. To increase the spin yarn orientation, the process uses a stress elevation by means of a fast cooling of the threads directly beneath the spinning nozzle. As is also indicated from the above patent specification (FIG. 1), within the framework of the method, a counterflow blowing is used. However, in this case, maximum capillary counts of 20 to 34 are disclosed.
A cooling process of this kind and other conventional quenching processes in general lead to nonhomogeneous yarn properties, and thus to an increased filament breakage rate. In particular, when using several hundred capillaries (e.g., more than 200), as the ordinary technician will know, multiple problems occur.
Nonhomogeneous yarn properties are obtained in the otherwise standard cooling processes, particularly when a relatively high and defined spin yarn orientation is required for the use of many capillaries. The latter pertains to special methods for the production of polyester yarns with low shrinkage and large modulus, the so-called low-shrinkage-high-modulus yarns (L.S.H.M. yarns). The nonhomogeneity problems thus increase with a greater the number of filaments at a given single titer. The single filament titer is between 1 and 20 dtex. U.S. Pat. No. 4,491,657 describes the production of a yarn with large modulus and low shrinkage. However, the described method with a conventional cooling technique cannot be transferred to a one-step spin-drawing process owing to the high required spinning speeds. In addition, in the use of a conventional cooling system, particularly when many capillaries are to be used, even in a two-step method (spinning-and drawing separately) the homogeneity of the yarns is limited.
From German Patent No. 3629731 and No. 3708168 it is already known how to achieve a good homogeneity in the mechanical properties. The problem of a simultaneously high molecular orientation in the spin material however is not found to be addressed in these publications. The provision of a high molecular orientation with simultaneous homogeneity of the high capillary spin yarn as a basis of a multifilament for specific use in L.S.H.M. is not found in either German Patent No. 3629731 or even in German Patent No. 3708168.
European Patent No. 0527134 pertains in general to the homogeneous production of filaments by use of a central quenching system. However, the possibilities for the production of yarns with low shrinkage and higher modulus are not recognized in this publication. Furthermore, clearly fewer than 150 capillaries are used in this publication.
In additional studies that form the basis of the present invention, it turned out that, in particular, certain combinations of spacer lengths (spacing between blow candle* and spinneret), lengths of the active and passive part of the central quenching system, the blown air speed, the blown air temperature, and the blown air profile produce definite improvements, specifically with regard to the production and/or the properties of L.S.H.M (low-shrinkage-high-modulus) yarns.